Top-Down Approach to Drawing Data Flow Diagrams

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The top-down approach to drawing data flow diagrams essentially involves moving “from the general to the specific” in terms of the detail level displayed (Kendall & Kendall, 2008, p. 220). Using information gathered from interviews, investigation, and personal observation, the systems analyst begins by developing a list of business activities. They then use the list to define the four elements of a data flow diagram, as follows:

§ External entities – send/receive data to/from the system

§ Data flows – the movement of data

§ Processes – the change or transformation of data

§ Data stores – a virtual storehouse where data can be added, examined, or retrieved

(Kendall & Kendall, 2008)

The analyst continues by creating a context diagram which only shows external entities and basic data flows into and out of the system. The context diagram contains a single process representing the whole system (Kendall & Kendall, 2008).

The analyst then draws Diagram 0, which is the next lower level from the context diagram. Diagram 0 “explodes” the context level diagram, providing a closer, more detailed view of the system. It typically includes three to nine processes and shows the major data stores which represent master files. At Diagram 0, inputs and outputs relative to the context level diagram remain unchanged but the data flows expand to lower levels with increased detail (Kendall & Kendall, 2008).

At this point the analyst proceeds to explode Diagram 0, adding a more detailed child diagram for each of Diagram 0’s processes. Each exploded Diagram 0 process is called a parent process, and the resultant diagram is called a child diagram. Past Diagram 0, entities are generally not shown (Kendall & Kendall, 2008).

There must be ...

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...es successively more detailed diagrams. The diagrams progress from the basic, context level diagram, to the more detailed Diagram 0, to the even more detailed child diagrams which branch from Diagram 0’s parent processes.

The analyst must check their child diagrams for errors, particularly to avoid creating unbalanced conditions where there are input and/or outputs which do not appear on the corresponding parent diagrams. The diagram progresses to a further level of detail, moving from a logical to a physical data flow diagram. Finally, the analyst partitions the physical diagram, creating groups of automated processes which will become programs, or, separating processes which for various reasons cannot or should not be grouped together.

Works Cited
Kendall, K. & Kendall, J. (2008). Systems Analysis and Design. Upper Saddle River, NJ: Pearson Custom Publishing.

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